This invention relates generally to integrated drive generators (IDG) used on aircraft that are hydraulically operated and in particular to a method and apparatus for maintaining the flow of hydraulic fluid to the IDG in the event that the aircraft experiences negative or zero xe2x80x9cgxe2x80x9d conditions.
An integrated drive generator, (IDG), is an integral unit having a constant speed drive variable transmission (CVT), or a hydrostat pump and motor, that converts variable speed rotary input from a shaft of an aircraft propulsion engine into a constant speed shaft drive which drives an electrical generator producing constant frequency three phase 400 Hz power. The transmission operates on hydraulic fluid or oil.
Referring to FIG. 1, an IDG 10 includes a casing 12 which acts as an oil sump by collecting hydraulic fluid 26 at its bottom. When hydraulic power is required to drive the CVT, oil from the sum is pumped to the CVT. In a negative xe2x80x9cgxe2x80x9d condition, the oil is forced from the bottom of the sump to the top and in a zero xe2x80x9cgxe2x80x9d condition the oil floats in the middle between the top and the bottom. Both of these conditions are referred to as adverse xe2x80x9cCgxe2x80x9d conditions and when either occur, the oil pressure at the bottom of the sump drops and the flow is interrupted. Such an interruption of flow or drop in oil pressure to the CVT will result in the CVT being unable to hold a constant generator speed which will cause an automatic shut down of the IDG.
Commercial aircraft experience zero or negative xe2x80x9cgxe2x80x9d conditions under a variety of circumstances such as severe weather or emergency maneuvers. Clearly, when these circumstances occur it is important that the CVT continue to maintain constant generator speed, otherwise the aircraft will lose electric power. As a consequence, IDGs used on commercial aircraft are typically required to operate normally for a duration of 15 seconds of zero or negative xe2x80x9cgxe2x80x9d forces.
One method used to meet this 15 second requirement is to provide a second pump for pumping oil from the top of the sump when negative xe2x80x9cgxe2x80x9d is experienced. This method has had only limited success, because mounted in the sump are a plurality of rotating components which inhibit the flow of oil from the bottom to the top. Instead of the oil flowing smoothly to the top, it gets flung around the casing by these rotating components. Further, this method does not address the zero xe2x80x9cgxe2x80x9d conditions where oil tends to float in the middle of the sump.
Accordingly, there is a need for a method and apparatus for maintaining for a period of at least 15 seconds the flow of hydraulic fluid to the IDG in the event that the aircraft experiences negative or zero xe2x80x9cgxe2x80x9d conditions.
An object of the present invention is to provide an apparatus for maintaining for a period of at least 15 seconds the flow of hydraulic fluid to the IDG in the event that the IDG experiences negative or zero xe2x80x9cgxe2x80x9d conditions.
Another object of the present invention is to provide a method for maintaining for a period of at least 15 seconds the flow of hydraulic fluid to the IDG in the event that the IDG experiences negative or zero xe2x80x9cgxe2x80x9d conditions.
The present invention meets these objectives providing an IDG hydraulic system that utilizes in flow series arrangement a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse xe2x80x9cgxe2x80x9d condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. A small accumulator of oil is used to make up for leakage in the system.
A method for providing hydraulic fluid to an integrated drive generator having a constant speed drive variable transmission comprising the steps of during normal xe2x80x9cgxe2x80x9d conditions pumping said fluid from a sump to said transmission and returning said fluid from said transmission to mix with the fluid from said sump and sensing an adverse xe2x80x9cgxe2x80x9d condition and in response thereto cease pumping from said sump and re-circulating the fluid returning from said transmission back to said transmission.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.